Sound Transit monitors big Beacon Hill sinkhole

Published 10:00 pm, Thursday, April 2, 2009

Sound Transit is monitoring a Beacon Hill neighborhood after a large sinkhole formed on a resident's property about 135 feet above the northbound light rail tunnel, the agency said Friday.

Obayashi Corp., the agency's contractor, sent crews to plug the hole with cement-like filler material. The property owner notified Sound Transit March 29 about the hole with concerns that tunnel construction caused it.

Tunnel boring was completed more than a year ago.

"The strange thing is we don't know exactly how it was formed. We would have had to have done extensive geological investigation into the hole, which would have incurred costs and taken time," Sound Transit spokesman Geoff Patrick said.

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"We're not aware of any reason to be concerned. That said, we're going to be closely monitoring the area around there."

The property is on 18th Avenue South, near South Lander Street. The hole formed in the residents' yard, but didn't damage their home, Patrick said.

Crews shut down 18th Avenue South between South Lander Street and Bayview Street to plug the hole and stabilize the ground, Sound Transit reported.

Two side-by-side tunnels, each a mile long, run through Beacon Hill as part of a $2.4 billion Link Light Rail line that is set to open in July, connecting downtown Seattle and Tukwila.

Sound Transit has been monitoring shifts in the ground with geotechnical instruments. After tunnel work completed, the soil shifted about a quarter-inch in the area, but no other holes have been reported, Patrick said.

The hole measured 18 inches at the mouth, but grew larger in width at the bottom, which was unusual, Patrick said.

"You wouldn't have seen anything from the street whatsoever, but it went down a long way," Patrick said.

Numerous trucks drove into the neighborhood Friday, from the early morning until the afternoon, according to residents. The trucks hauled Controlled Density Fill, a "self-compacting, cement-like" material to be pumped into the hole to stabilize the ground, according to Sound Transit.